It is widely known that ultraviolet light radiation, whether from a natural source such as the sun or an artificial source such as interior lighting, can accelerate both the physical and chemical breakdown or deterioration of various polymeric resin materials. For example, it is known that polyolefin, acrylic, polycarbonate, polyvinylaromatic, polyester, polyamide and polyvinylchloride resin materials are all susceptible, to one degree or another, to both physical and chemical degradation when exposed to ultraviolet light radiation.
To protect these polymeric resin materials against deleterious effects of ultraviolet light radiation, a number of commercial ultraviolet light absorbing additive agents have been developed. In general, these commercial light absorbing agents will provide the stability desired in otherwise ultraviolet light degradable polymeric resin materials either by absorbing the ultraviolet light radiation directly or by harmlessly dissipating this particular light energy through intermolecular energy processes.
In the main, the most widely employed commercial ultraviolet light absorbing additive agents are organic in nature and generally can be grouped into five basic categories. These categories include (1) hydroxybenzophenones, (2) hydroxyphenylbenzotriazoles, (3) salicylates, (4) aryl substituted acrylates and (5) aminobenzoates. While these organic additives function as ultraviolet light absorbents, they are costly to produce and can add significantly to the cost of formulated polymeric resin materials containing them.
A less expensive commercially available material for use as an ultraviolet light absorbing additive agent in certain types of polymeric resin materials is the inorganic metal oxide, titanium dioxide. Its use for such purposes is known from U.S. Pat. No. 4,619,957 wherein it is disclosed that titanium dioxide is added to poly(vinylchloride) resin formulations not only to provide pigmentation in the formulations but also to impart improved ultraviolet light stability to such formulations. It further is disclosed in this patent that one problem with the use of titanium dioxide for this purpose is that due to the high loadings required and the strong tinting strength of the titanium dioxide, it is not possible to fabricate products from such resin formulations in colors other than white or light pastel colors. To overcome this problem, the solution proposed in this patent is to replace a portion of the titanium dioxide in such resin formulation with certain ultraviolet light absorbing benzoate compounds.
According to the disclosures in the above patent, reduction of the titanium dioxide levels in such resin formulations allows for the manufacture of fabricated products in darker shades or colors. The added benzoate compounds function to maintain the stability of the fabricated products, formerly provided by the high levels of titanium dioxide, against degradation by ultraviolet light radiation.
Another drawback to the use of titanium dioxide is that this pigment does not appear to be universally suited for use as an ultraviolet light absorbing additive agent in all ultraviolet light degradable polymeric resins. For example, when added to ultraviolet light degradable polyamides (e.g., nylons) as a delusterant, titanium dioxide is believed to act as a photosensitizer and, in effect, increases the rate of photodegradation of the polyamide particularly at the longer wavelengths in the ultraviolet light resin. Thus, to provide ultraviolet light stability to a polyamide containing titanium dioxide it is usual practice to further add to the polyamide resin formulation a stabilizing compound such as a manganese salt together with other additives such as hypophosphorous acids, phosphites, phosphates, and the like. Kirk-Othmer Encyclopedia of Chemical Technology, Vol. 16, page 81, 2ed. (1968).
It is clear from the above that ultraviolet light stabilized polymeric resin compositions have been prepared from a wide variety of ultraviolet light degradable polymeric resin materials and ultraviolet light absorbing additive agents. However, a need still exists for ultraviolet light stabilized polymeric resin compositions that are more economical to produce than those based on the use of the aforementioned organic ultraviolet light absorbing agents and which compositions are not bothered by problems, such as those mentioned above, associated with the use of inorganic ultraviolet light absorbing agents such as titanium dioxide.